Applications of dielectric resonators in filter design have become more and more popular due to impressive advantages, such as small size, low weight, low loss (high Q), and common commercial availability. Dielectric resonators employed in filters could be utilized in a variety of modes, such as TE, TM, and HEM (hybrid electromagnetic) modes. Filters using resonators employing dual hybrid mode exhibit a symmetric bandpass response. Such resonators are mainly used in satellite communications. However, in applications where an asymmetric bandpass response is desired, such as mobile telephone communication, hybrid mode resonators are difficult to implement. Multi mode resonators having specific geometric forms are often complicated to manufacture. Due to drawbacks with hybrid dual mode and the low Q in conventional TM mode structure, the TE.sub.010 mode have been extensively used in most of the high Q dielectrically loaded filters.
Accordingly, TE mode dielectric resonators are frequently used in mobile telephone communication systems. Because a TE.sub.010 mode dielectric resonator body needs to be placed symmetrically in the center of the cavity, in order to obtain highest possible Q, a temperature stable and low loss mount to support the resonator is required. The selection of material and design of this support is critical to the performance of the filter and becomes a significant feature in the manufacture of the product. A dielectric resonator with such a support is disclosed in U.S. Pat. No. 5 612 655.
Tuning of the resonance frequency is provided by a tuning screw extending through the top cover. Because of strong magnetic field intensity in the center, a big metallic disk is typically soldered to the end of the screw. Due to limited tuning range, and following the demand for center symmetric mounting, the required coarse frequency modification of the resonator is mostly done by reducing the radius dimension by grinding. The above operations are costly and time consuming in the manufacture of the resonator.
A resonator of this kind is disclosed in U.S. Pat. No. 4 963 841. In this resonator the support has been replaced by a substrate in order to decrease vibration sensitivity. Through the selection of a substrate having a dielectric constant substantially greater than that of air, the dielectric resonator can be bonded to tho substrate, which in turn is bonded to a surface of the cavity, whereas the resonator is electrically centered in the cavity.
In filter application, coupling between TE.sub.010 mode cavities is provided through an iris, and the adjustment is accomplished by a coupling adjusting means (a screw, wire or the like) placed tangential to the electric field. In this case (TE mode), a coupling adjusting means extends horizontally in the plane of the iris, which is difficult to implement and makes it difficult to access. When implementing a TE mode broadband filter, problems will arise with the adjacent TM mode since this mode couples strongly via the used apertures. This can easily lead to spurious response with adjacent TM modes.
Furthermore, due to the fact that the material of the support is d dielectric material, it is very poor in conducting heat. Thus, in high power applications, the temperature of the dielectric resonator can be very high, which may cause serious problems.
In conventional TM mode design, a dielectric resonator is placed on a substrate. The substrate can be placed on the base of a cavity. This offers significant size reduction, but poor unloaded Q (Q.sub.u) compared to TE.sub.010 mode (Q.sub.u =5000 for TM.sub.010 and Q.sub.u =13000 for TE.sub.010 at 1,9 GHz and .epsilon.=30 to 40).
However, in filter applications, the use of TM mode resonators is preferable for a number of reasons, such as good coupling between TM modes through apertures, easy tuning and a good spurious response (attenuation of unwanted modes). For example, the coupling adjusting means, discussed above in connection with TE mode filters, can in TM mode filters be axially mounted, which is easily made in the lid of the filter unit.
U.S. Pat. No. 4 613 838 discloses a dielectric resonator employing a TM mode. It includes a cavity resonator and a columnar inner dielectric member accommodated within the cavity. The dielectric member is in contact at its both ends with inner electrode surfaces of the cavity. This document is regarded to disclose the prior art closest to the invention, since a dielectric resonator employing a TM mode is disclosed.